The present invention relates to a real image type zoom finder used for a viewfinder of a camera, specifically to a real image type zoom finder which can secure a large back focal length of an objective lens though it is compact.
Recently, the real image type zoom finder is widely used as a zoom finder because its variable magnification ratio can be easily increased and it is advantageous in clearness of a field diaphragm, comparing to a virtual image type one. In a real image type zoom finder, an image erecting member for erecting an object image is necessary, therefore, a real image type zoom finder which is compact and has a long back focal length of an objective lens, is required for arrangement of an image erecting member.
As an objective lens of the real image type zoom finder, an objective lens having a structure in which at least 2 lens groups are moved for correcting a change of diopter during a magnification varying operation, is widely used. The objective lens having more than 2 moving groups, is divided into an objective lens having a structure in which a lens group closest to the object is moved, and an objective lens having other structures. An objective lens having a structure in which a lens group closest to the object is moved, is not preferable because dusts easily enter a viewfinder structurally. Further, in order to appropriately secure a back focal length of the objective lens, it is desirable to position a lens group having negative refractive power at a position closest to the object side of the objective lens.
In order to satisfy these requirements, examples of the real image type zoom finder having an object lens, which is composed of at least 3 lens groups of a fixed lens group having negative refractive power at a position closest to an object side, and 2 lens groups movable during a magnification varying operation, and in which a back focal length is comparatively long, are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 234103/1996, 248315/1996, etc. In these conventional examples, as a back focal length of the objective lens, a sufficient space can be secured for 2 reflection surfaces of a roof mirror or a Porro prism, although magnification of a viewfinder is 0.4 at a wide angle end.
However, recently, while size reduction of overall camera is intended, a requirement for a real image type zoom finder is increased in which a larger back focal length of an objective lens can be secured, for example, by means of a method in which a reflection element having 3 reflection surfaces is arranged between an objective lens and a field diaphragm, for a convenience of arrangement of other parts.